fix a PHP Notice

apps/calendar/ajax/settings/timezonedetection.php

@@ -8,10 +8,13 @@
  
 OC_JSON::checkLoggedIn();
 OC_JSON::checkAppEnabled('calendar');
+if(array_key_exists('timezonedetection', $_POST)){
 	if($_POST['timezonedetection'] == 'on'){
 		OC_Preferences::setValue(OC_USER::getUser(), 'calendar', 'timezonedetection', 'true');
 	}else{
 		OC_Preferences::setValue(OC_USER::getUser(), 'calendar', 'timezonedetection', 'false');
 	}
 	OC_JSON::success();
-
+}else{
+	OC_JSON::error();
+}
\ No newline at end of file